Variability of intergranular friction and its role in DEM simulation of direct shear of an assembly of rapeseeds

A method for determining the coefficient of intergranular friction between pairs of metallic and organic objects of nearly spherical shape was elaborated. The method consists of simultaneous determination of both the force of friction and relative displacements of a pair of grains under normal load. This method was verified by measuring the friction force between pairs of steel plates and pairs of steel beads with a diameter of 5 mm. The optimum conditions for ensuring accuracy and repeatability of these measurements were established. Coefficients of friction of steel objects and pea, wheat and rapeseed grains at equilibrium moisture content were determined. It was found that the scatter of friction force was the highest for pea, wheat and rapeseed grains due to variability of shape, roughness of surface and irreversible deformation at the points of contact. Numerical simulations using discrete element method were performed to examine the influence of variability which the coefficient of intergranular friction had on material behaviour in a direct shear test. Two-dimensional simulations were performed assuming an assembly of 4000 circular particles having the material properties of rapeseeds with three different levels of standard deviations of mu(pp). Variation in the standard deviation of mu(pp) was found to influence markedly the stress strain characteristics while the strength of the assembly (or steady state value of stress) remained constant.